Vaporization of metals and metalloids



March 12, 1968 A. HlLLlARD 3,373,260

VAPORIZATION OF METALS AND METALLOIDS Filed April 26, 1965 2Sheets-Sheet 1 16 FIG.1

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VAPORIZATION OF METALS AND METALLOIDS Filed April 26, 1965 2Sheets-Sheet 2 we My United States Patent O 3,373,260 VAPORIZATION FMETALS AND METALLOIDS Alfred Hilliard, London, England, assignor toSociete des Poudres Metalliques et des Alliages Speciaux Ugine- Carbone,Paris, France Filed Apr. 26, 1965, Ser. No. 450,730 Claims priority,application Great Britain, Apr. 30, 1964, 17,963/ 64 13 Claims. (Cl.219-275) ABSTRACT OF THE DISCLOSURE The disclosure provides anelectrical heater which supports a plug for evaporating aluminium orother volatilisable material, the plug being contained in a recess inthe heater which recess is smaller at the upper part than the lower partso that the plug will not rise out of the recess due to electromagneticaction.

This invention relates to the evaporation of metals or metalloids, e.g.aluminium, chromium, nickel and silicon. Such evaporation is requiredfor the plating of articles by vapor deposition.

One method for effecting evaporation of metals and metalloids comprisesthe use of an electrically conducting bar or elongated crucible or'boatcalled the evaporation source, clamped between two electrodes, throughwhich current is passed which heats the source by electrical resistanceso that metal fed on the latter is evaporated, for example as describedin British Patents Nos. 770,751 and 770,752.

Many metals and metalloids are highly corrosive at the temperatures ofevaporation.

The characteristics best suited for the heating function, such assuitable electrical resistance, mechanical strength, and thermal andmechanical flexibility, are often not found in the constructionalmaterials suited for the function of best chemical resistance to thesubstances which are to be evaporated.

It is, therefore, often advantageous to separate these two functions,for example as described in British Patent No. 770,753, by using anelectrical resistance heating element, hereafter, referred to as theheater having a recess in which is inserted a removable refractory plugor similar insert, hereafter called the plug.

The heater may be made from graphite or from refractory carbide,'boride, nitride, oxide or a refractory metal like tungsten ormolybdenum or mixtures thereof, or other refractory material. The plugmay be made from a refractory carbide, boride, nitride, oxide (forexample, tantalum carbide) or mixtures thereof, or other materialresistant to attack by the material to be evaporated.

The material (which term hereafter is intended to include metals andmetalloids) to be evaporated is fed to the centre of the top surface ofthe plug where it collects as a pool of molten metal and vaporization isefiected by heat conducted to the metal pool from the surface of theheater. Solid metal is commonly fed in the form of a wire to the plug.

One reason for keeping the plug as a detachable insert,

is that the metal to be evaporated usually attacks the article fromwhich the material evaporates. Economies are, therefore, effected byusing a multiplicity of plugs in the same heater or, in appropriatecases, the heaters can be made from a cheap substance such as graphiteand a multiplicity of heaters may be used with the same plug.

In existing arrangements, the upper face of the plug is either flushwith, or above the upper face of the heater (e.g. FIGURE 3 in BritishPatent No. 770,753).

I have found that attack by the material on the plug ICE is concentratedin areas where liquid material collects as a molten pool on the plugsurface, but that attack by vapor of the material on the plug is eitherabsent or small and generally less than attack by the liquid material.Liquid material may also attack the heater especially by contact withthe surface of the heater in the recess.

' According to the present invention, a heater suitable for evaporatingvolatilisable material has a recess to receive a plug from which thematerial can be evaporated, said recess being smaller at the top than atthe bottom thereof.

This invention makes it possible to use a plug which tends to levitatedue to an electromagnetic field produced by electrical heating of theheater. The operating temperature is such as is necessary to vaporizethe material. For example, if the material is aluminium, a suitabletemperature of said zone is in the region of l650-1700 C. for whichpurpose the heater may be raised to a temperature of about 1900 to 2000C.

In order to create this zone, it is necessary that the top of the plugshall be below the plane of the surface of the heater at the top edgesof the recess. The distance between the top of the plug and the top ofthe recess may be varied within wide limits, e.g. 0.01 to 0.8 of thedepth of the recess, or in most cases 0.1 to 0.3. The upper face of theplug may, for example, be flat or dome shaped.

Although the invention minimises contact of liquid material with theplug, such contact may accidentally occur, e.g. on starting up or due toirregular material feed or current fluctuation, and the presentinvention also provides special forms of plug which avoids moltenmaterial reaching the heater.

It is important to preserve the plug surface substantially intact, i.e.to prevent or minimise crater formation (namely, the opposite of theeffect shown in FIGURE 1 of British Patent No. 770,753). If a crater isformed in a plug, the material to be evaporated is shielded from heatradiated by the heater, and receives then substantially only conductedheat which induces accumulation of molten material in the crater and aprogressively increasing rate of corrosion. The material may be fedcontinuously or intermittently towards a recess in an electricallyheated heater which recess contains a plug, the feed and the temperaturebeing such that the solid material is not contacted with the plug but isvaporized above the plug.

According to another aspect of the present invention, the recess in theheater is shaped to provide an abutment against which the plug cancontact to prevent its being ejected from the recess by the magneticfield of the electric current used for heating. This :aspect of theinvention permits the use of lightweight compounds for the plug, forexample titanium di-boride. Earlier methods excluded the use oflightweight plug materials although some of them, e.g. titaniumdi-boride, have very desirable resistance to the attack by metals atelevated temperatures, and they may also be cheaper.

If a lightweight plug is used, the plug is inserted into the heaterorifice from below and is held in position by a suitable support, whichsupport need not be a conduc-- tor of electricity. This triplearrangement, which separates the functions of the plug, the heater andthe support, has the following added advantages.

The mechanical load is carried by the support which can, therefore, beselected in a material of best mechanical strength at lowest costwithout the limitation of having to allow for suitable electricalcharacteristics.

The heater, which is carried by the support, can be selectedsubstantially on the basis of most favorable electrical characteristics.

A particular advantage of dividing the heater in this way is that theresulting geometrical form becomes then much more resistant to thermalstresses, namely improved thermal flexibility, which manifests itself inpractice by a multiple operational life.

The arrangement will now be further described by way of example withreference to the accompanying diagrammatic drawings, wherein:

FIGURE 1 is a sectional view of a heater made in accordance with theinvention together with the plug;

FIGURES 2 and 3 show modifications to be described;

FIGURE 4 is a section on line 4-4 on FIGURE 3;

FIGURE 5 is a plan view showing a modified form of heater;

FIGURE 6 is a section AA on FIGURE 5, showing also the support; and

FIGURES 7 and 8 show modified shapes of plugs.

Referring first to FIGURE 1, a bar shaped heater 10 made of graphite isprovided with a conical recess 11, the upper end of which is smallerthan its bottom end. In the recess 11 is a plug 13 which is cylindricalin plan and has a height less than that of the recess so as to leave ahigh temperature zone 14 above the plug Within the recess. A rod or wire16, of material to be vaporized, is fed into the hot zone 14 and is thusvaporized by radiated heat. The function of the plug is relegated tothat of the removable safety insert in the heater so that material,which accidentally may not have been vaporized, will fall on to thedetachable plug insert instead of contacting the heater with consequentrisk of attack on the heater.

The heater permits the use of mechanically light mate rials (for exampletitanium di-boride) for the plug inserts.

The plug 13 is inserted through the bottom of the heater orifice 11. Ifan electromagnetic field induces levitation of the plug, the plug isprevented from being ejected from the heater or orifice.

The heater is made in two parts, viz the heater part 10 having therecess 11 and a lower part 25 which may be made of stronger material(e.g. vitreous carbon, alumina, or the like) and serves to support thepart 10 and also to support the plug 13. The parts 10 and 25 may beclamped together by any suitable means, or support 25 only may beclamped with the heater 10 sitting on support 25.

FIGURE 2 shows a modified arrangement also having a supporting member25. This permits also versatility, with respect to the wall thickness ofthe heater 10 around the orifice 11 to produce highest electricalresistance in the desired areas, namely, immediately above the plug 13.The supporting member 25 has a projection 28 engaging a correspondingslot in the heater part 10 in the region of the recess. Recess 11 ispartly in the heater and partly in the member 25.

The supporting member 25 which carries the plug 13, can also be held inposition by other means. For example, FIGURE 3 shows a side view of suchan arrangement and FIGURE 4 is a transverse sectional view of thearrangement of FIGURE 3. In this example the heater has at the bottomend, protruding ribs 27 over which upstanding flanges 26 at the edges ofthe supporting member 25 can be slid.

FIGURES 5 and 6 show an arrangement similar to that of FIGURE 1 but withthe orifice 11 cylindrical in the lower part and trunco-conical onlynear the upper end. One object of this arrangement is to ensure that thedistance. between the plug 13 and the orifice 11 is either substantiallyuniform 'or slightly greater towards the top end, so as to minimise anytendency for vapors to be trapped near the lower end of the orifice 11.

A thin disc can, if required, be inserted below the plug. This disc canbe made from a suitable mate-rial, e.g. vitreous carbon.

The plug, for the purpose of the present invention, may be cylindrical,cubic or of other shape.

Heating may be effected, if desired, by means other than electricalresistance heating, e.g. by high frequency heating.

As shown in FIGURE 5 the walls 12 may be thinner than the remainder ofthe bar 10 in plan view.

I claim:

1. An electrical resistance heater suitable for evaporatingvolatilisable material having a recess, a plug disposed in said recess,from which plug the material can be evaporated, said recess beingsmaller at the top than at the bottom thereof to prevent expulsion ofthe plug by levitation.

2. A heater as claimed in claim 1, wherein the recess is frusto-conical.

3. A heater as claimed in claim 1, wherein the recess is frusto-conicalat the upper part thereof and cylindrical at the lower part thereof.

4. A heater as claimed in claim 1, wherein the plug is formed of alight-weight material such as titanium diboride.

5. A heater as claimed in claim 1, having the recess extendingcompletely therethrough so that the plug can be inserted from below anda separate supporting member is provided to support the plug in therecess, the heater being made of a material suitable for electricalresistance heating while the supporting member is made of a differentmaterial having better mechanical properties than the heater material.

6. A heater as claimed in claim 5, wherein the heater and the supportingmember have inter-engaging ribs and flanges for sliding engagement.

7. A heater as claimed in claim 1, in combination with an evaporationsource in the form of a plug in the recess of less height than therecess so that a zone for high temperature evaporation is provided inthe recess above the plug so as to minimise or avoid formation of moltenmaterial on the plug.

8. The combination claimed in claim 7, wherein the distance between thetop of the plug and the top of the recess is from 0.1 to 0.3 of thedepth of the recess.

9. The combination claimed in claim 7, wherein the top of the plug isdome shaped.

10. The combination claimed in claim 7, wherein the plug has an annulardepression in its upper end.

11. The combination claimed in claim 7, wherein the recess in the heaterextends completely therethrough and the heater has projecting means atthe lower part of the recess to support the plug.

12. A heater as claimed in claim 1, wherein the wall thickness of theheater at the recess is reduced at the lower part thereof.

13. A heater as claimed in claim 5, wherein the supporting member has aprojecting part extending into the heater in the region of the recess,the recess being partly in the heater and partly in the supportingmember.

References Cited UNITED STATES PATENTS 2,772,318 11/1956 Holland 13-252,793,609 5/1957 Tzu En Shen et a1. 118-49 2,902,525 9/1959 Barker118-49 X 2,940,873 6/1960 Toohig 118--49 X 3,017,851 1/1962 Krause 118493,057,936 10/1962 Hill 1325 3,271,561 9/1966 Fiedler et al 219--271RICHARD M. WOOD, Primary Examiner.

C. L. ALBRITTON, Assistant Examiner.

